This invention relates generally to devices for controlling the phase and polarization of microwave signals and more particularly to a combination phase shifter/polarization switch adapted to operate over an octave bandwidth.
As is known in the art, a collimated beam of radio frequency energy may be formed and steered by controlling the phase of the energy radiated from each one of a plurality of antenna elements in an array thereof. The two principal means of providing electronic control of the phase of microwave signals are realized by diode and ferrite phase shifters. Ferrite phase shifters present a nearly uniform propagation medium to microwave signals passing therethrough, and therefore they are capable of operating over a relatively wide bandwidth. However, ferrite phase shifters are nonreciprocal and in situations where polarization diversity is desired, such as in ECM applications, additional elements such as nonreciprocal polarizers and switchable quarter-wave plates must be combined with the ferrite phase shifter. The use of such nonreciprocal polarizers and switchable quarter-wave plates, while adequate in many applications, has been found to be inadequate when it is required that the devices operate over a relatively wide frequency band. This is so because the bandwidth of the phase shifter will be limited to that of the switchable quarter-wave plate and will, therefore, be limited to approximately a 20 percent band. Such a device is, therefore, impractical in applications wherein it is required that the devices have greater than an octave bandwidth as in ECM and ECCM applications. Additionally, the extra size and weight required by ferrite phase shifters to provide polarization diversity makes the use of such devices impractical in airborne phased array applications.
As is known in the art, diode phase shifters are attractive for airborne phased array applications because they are lightweight, temperature insensitive, and are capable of high speed switching rates. Diode polarizers may be formed from 90 and 180 degree phase bits and therefore polarization diversity may be readily integrated into the phase shifter design. Diode phase shifters are, however, relatively bandwidth limited. One known diode phase shifter is described in an article entitled "A Low Cost P-I-N Diode Phase Shifter For Airborne Phased Array Antennas" by F. G. Terrio, R. J. Stockton and W. D. Sato, IEEE Transactions on Microwave Theory and Techniques, June 1974, pages 688-692. In such phase shifter pin diode chips were used as the switching elements, and the useful bandwidth of such device, allowing a maximum phase error of .+-. 22.5.degree., is 40 percent. In the same device for a maximum permissible phase error of 35 10.degree., the bandwidth is about 30 percent.
In airborne applications it is desirable to employ hermetically sealed semiconductor packages so that potting or sealing is not required to protect the diodes. The use of packaged diodes further reduces the bandwidth of the phase shifter due to the parasitic reactances which the diode package adds to the circuit, as is reported in an article entitled "Diode Phase Shifters For Array Antennas" by J. F. White, IEEE Transactions on Microwave Theory and Techniques, June 1974, pages 658-674. Additionally, high frequency phase shifter circuits, fabricated using stripline or microstrip techniques, usually employ ground plane spacings less than 0.100 inches in order to suppress higher order modes. Since the length of a standard diode package is greater than twice the ground plane spacing, the use of packaged diodes in high frequency phase shifter circuits has been impractical. Obviously, such considerations make it extremely difficult to provide a diode phase shifter employing packaged pin diodes and having an octave bandwidth.